1. Chapter 4 Writing Classes

2. © 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes The programs we’ve written in previous examples have used classes defined in the Java standard class library Now we will begin to design programs that rely on classes that we write ourselves The class that contains the main method is just the starting point of a program True object-oriented programming is based on defining classes that represent objects with well- defined characteristics and functionality

3. © 2004 Pearson Addison-Wesley. All rights reserved4-3 A sample problem Write a method that will throw 2 Dice with varying number of sides a specified amount of times and reports how many times we got a snake eyes (both dice showing 1) For example numSnakeEyes(6, 13, 100) should return the number of snake eyes after throwing a 6 sided Die and 13 sided Die 100 times. We will first show a structured approach

4. © 2004 Pearson Addison-Wesley. All rights reserved4-4 Structured Die static Random rand = new Random(); static int roll(int numSides) { return 1 + rand.nextInt(numSides); } static int numSnakeEyes(int sides1, int sides2, int numThrows) { int count = 0; for(int i = 0; i < numThrows; i++) { int face1 = roll(sides1); int face2 = roll(sides2); if (face1 == 1 && face2 == 1) count++; } return count; }

5. © 2004 Pearson Addison-Wesley. All rights reserved4-5 Object Oriented Approach In OOP, we first focus on the main actors, not how things are done. The main actors here are Die objects. We need to define a Die class that captures the state and behavior of a Die. We can then instantiate as many die objects as we need for any particular program

6. © 2004 Pearson Addison-Wesley. All rights reserved4-6 Classes A class can contain data declarations and method declarations int size, weight; char category; Data declarations Method declarations

7. © 2004 Pearson Addison-Wesley. All rights reserved4-7 Data and Methods The values of the data define the state of an object created from the class The functionality of the methods define the behaviors of the object For our Die class, we might declare an integer that represents the current value showing on the face, and another to keep the number of faces One of the methods would “roll” the die by setting that value to a random number between one and number of faces, we also need methods to give us information about our object.

8. © 2004 Pearson Addison-Wesley. All rights reserved4-8 Classes We’ll want to design the Die class with other data and methods to make it a versatile and reusable resource Any given program will not necessarily use all aspects of a given class See RollingDice.java (page 157)RollingDice.java See Die.java (page 158)Die.java

9. © 2004 Pearson Addison-Wesley. All rights reserved4-9 public class Die { private int numFaces; // maximum face value private int faceValue; // current value showing on the die // Constructor: Sets the initial face value. public Die(int _numFaces) { numFaces = _numFaces; roll(); } // Rolls the die public void roll() { faceValue = (int)(Math.random() * numFaces) + 1; } // Face value setter/mutator. public void setFaceValue (int value) { if (value <= numFaces) faceValue = value; }

10. © 2004 Pearson Addison-Wesley. All rights reserved4-10 Die Cont. // Face value getter/accessor. public int getFaceValue() { return faceValue; } // Face value getter/accessor. public int getNumFaces() { return numFaces; } // Returns a string representation of this die. public String toString() { return “number of Faces “ + numFaces + “current face value “ + faceValue); }

11. © 2004 Pearson Addison-Wesley. All rights reserved4-11 The new Version static int numSnakeEyes(int sides1, int sides2, int numThrows) { Die die1 = new Die(sides1); Die die2 = new Die(sides2); int count = 0; for(int i = 0; i < numThrows; i++) { die1.roll(); die2.roll(); if (die1.getFaceValue == 1 && die2.getFaceValue == 1 ) count++; } return count; }

12. © 2004 Pearson Addison-Wesley. All rights reserved4-12 Using Die class in general Die die1, die2; int sum; die1 = new Die(7); die2 = new Die(34); die1.roll(); die2.roll(); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); die1.roll(); die2.setFaceValue(4); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); sum = die1.getFaceValue() + die2.getFaceValue(); System.out.println ("Sum: " + sum); sum = die1.roll() + die2.roll(); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); System.out.println ("New sum: " + sum);

13. © 2004 Pearson Addison-Wesley. All rights reserved4-13 The Die Class The Die class contains two data values  numFaces that represents the maximum face value  an integer faceValue that represents the current face value The roll method uses the random method of the Math class to determine a new face value There are also methods to explicitly set and retrieve the current face value at any time

14. © 2004 Pearson Addison-Wesley. All rights reserved4-14 The toString Method All classes that represent objects should define a toString method The toString method returns a character string that represents the object in some way It is called automatically when an object is concatenated to a string or when it is passed to the println method

15. © 2004 Pearson Addison-Wesley. All rights reserved4-15 Constructors As mentioned previously, a constructor is a special method that is used to set up an object when it is initially created A constructor has the same name as the class The Die constructor is used to set the number of faces value of each new die object to a user defined value (passed as a parameter) We examine constructors in more detail later

16. © 2004 Pearson Addison-Wesley. All rights reserved4-16 Data Scope The scope of data is the area in a program in which that data can be referenced (used) Data declared at the class level can be referenced by all methods in that class Data declared within a method can be used only in that method Data declared within a method is called local data

17. © 2004 Pearson Addison-Wesley. All rights reserved4-17 Local and Class scope public class X{ private int a; // a has class scope, can be seen from // anywhere inside the class …. public void m() { a=5; // no problem int b = 0; // b is declared inside the method, local scope ….. } // here variable b is destroyed, no one will remember him public void m2() { a=3; // ok b = 4; // who is b? compiler will issue an error }

18. © 2004 Pearson Addison-Wesley. All rights reserved4-18 Instance Data The faceValue variable in the Die class is called instance data because each instance (object) that is created has its own version of it A class declares the type of the data, but it does not reserve any memory space for it Every time a Die object is created, a new faceValue variable is created as well The objects of a class share the method definitions, but each object has its own data space That's the only way two objects can have different states

19. © 2004 Pearson Addison-Wesley. All rights reserved4-19 Hotel room example public class X { int a; int b; void m1 () { System.out.println(a); m2(); } void m2() { System.out.println(b); } a=3 b=4 o1 a=1 b=2 o2 o1.m1() o2.m1()

20. © 2004 Pearson Addison-Wesley. All rights reserved4-20 Instance Data We can depict the two Die objects from the RollingDice program as follows: die1 5 faceValue die2 2 faceValue Each object maintains its own faceValue and numFaces variable, and thus its own state numFaces 6 9

21. © 2004 Pearson Addison-Wesley. All rights reserved4-21 Coin Example Write a program that will flip a coin 1000 times and report the number of heads and tails Flips two coins until one of them comes up heads three times in a row, and report the winner.

22. © 2004 Pearson Addison-Wesley. All rights reserved4-22 Coin Class public class Coin { private final int HEADS = 0; private final int TAILS = 1; private int face; public Coin () { flip(); } public void flip () { face = (int) (Math.random() * 2); } public boolean isHeads () { return (face == HEADS); } public String toString() { String faceName; if (face == HEADS) faceName = "Heads"; else faceName = "Tails"; return faceName; }

23. © 2004 Pearson Addison-Wesley. All rights reserved4-23 Count Flips final int NUM_FLIPS = 1000; int heads = 0, tails = 0; Coin myCoin = new Coin(); // instantiate the Coin object for (int count=1; count <= NUM_FLIPS; count++) { myCoin.flip(); if (myCoin.isHeads()) heads++; else tails++; } System.out.println ("The number flips: " + NUM_FLIPS); System.out.println ("The number of heads: " + heads); System.out.println ("The number of tails: " + tails);

24. © 2004 Pearson Addison-Wesley. All rights reserved4-24 FlipRace // Flips two coins until one of them comes up // heads three times in a row. public static void main (String[] args) { final int GOAL = 3; int count1 = 0, count2 = 0; // Create two separate coin objects Coin coin1 = new Coin(); Coin coin2 = new Coin(); while (count1 < GOAL && count2 < GOAL) { coin1.flip(); coin2.flip(); // Print the flip results (uses Coin's toString method) System.out.print ("Coin 1: " + coin1); System.out.println (" Coin 2: " + coin2); // Increment or reset the counters count1 = (coin1.isHeads()) ? count1+1 : 0; count2 = (coin2.isHeads()) ? count2+1 : 0; } // Determine the winner if (count1 < GOAL) System.out.println ("Coin 2 Wins!"); else if (count2 < GOAL) System.out.println ("Coin 1 Wins!"); else System.out.println ("It's a TIE!"); }

25. © 2004 Pearson Addison-Wesley. All rights reserved4-25 UML Diagrams UML stands for the Unified Modeling Language UML diagrams show relationships among classes and objects A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods) Lines between classes represent associations A dotted arrow shows that one class uses the other (calls its methods)

26. © 2004 Pearson Addison-Wesley. All rights reserved4-26 UML Class Diagrams A UML class diagram for the RollingDice program: RollingDice main (args : String[]) : void Die faceValue : int numFaces: int roll() : int setFaceValue (int value) : void getFaceValue() : int toString() : String

27. © 2004 Pearson Addison-Wesley. All rights reserved4-27 Encapsulation We can take one of two views of an object:  internal - the details of the variables and methods of the class that defines it  external - the services that an object provides and how the object interacts with the rest of the system From the external view, an object is an encapsulated entity, providing a set of specific services These services define the interface to the object

28. © 2004 Pearson Addison-Wesley. All rights reserved4-28 Encapsulation One object (called the client) may use another object for the services it provides The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished Any changes to the object's state (its variables) should be made by that object's methods We should make it difficult, if not impossible, for a client to access an object’s variables directly That is, an object should be self-governing

29. © 2004 Pearson Addison-Wesley. All rights reserved4-29 Encapsulation An encapsulated object can be thought of as a black box -- its inner workings are hidden from the client The client invokes the interface methods of the object, which manages the instance data Methods Data Client

30. © 2004 Pearson Addison-Wesley. All rights reserved4-30 Visibility Modifiers In Java, we accomplish encapsulation through the appropriate use of visibility modifiers A modifier is a Java reserved word that specifies particular characteristics of a method or data We've used the final modifier to define constants Java has three visibility modifiers: public, protected, and private The protected modifier involves inheritance, which we will discuss later

31. © 2004 Pearson Addison-Wesley. All rights reserved4-31 Visibility Modifiers Members of a class that are declared with public visibility can be referenced anywhere Members of a class that are declared with private visibility can be referenced only within that class Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package

32. © 2004 Pearson Addison-Wesley. All rights reserved4-32 package s.t; public class A { private int pv; int d; public int pb; m(…) { pv = 0; // OK d = 0; // OK pb = 0; // OK package s.t; public class B { … m(…) { A a = new A(..); a.pv = 0; // ERROR a.d = 0; // OK a.pb = 0; // OK package s.u; public class C { … m(…) { A a = new A(..); a.pv = 0; // ERROR a.d = 0; // ERROR a.pb = 0; // OK

33. © 2004 Pearson Addison-Wesley. All rights reserved4-33 Visibility Modifiers Public variables violate encapsulation because they allow the client to “reach in” and modify the values directly Therefore instance variables should not be declared with public visibility It is acceptable to give a constant public visibility, which allows it to be used outside of the class Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed

34. © 2004 Pearson Addison-Wesley. All rights reserved4-34 Visibility Modifiers Methods that provide the object's services are declared with public visibility so that they can be invoked by clients Public methods are also called service methods A method created simply to assist a service method is called a support method Since a support method is not intended to be called by a client, it should not be declared with public visibility

35. © 2004 Pearson Addison-Wesley. All rights reserved4-35 Visibility Modifiers publicprivate Variables Methods Provide services to clients Support other methods in the class Enforce encapsulation Violate encapsulation

36. © 2004 Pearson Addison-Wesley. All rights reserved4-36 Accessors and Mutators Because instance data is private, a class usually provides services to access and modify data values An accessor method returns the current value of a variable A mutator method changes the value of a variable The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value They are sometimes called “getters” and “setters”

37. © 2004 Pearson Addison-Wesley. All rights reserved4-37 Mutator Restrictions The use of mutators gives the class designer the ability to restrict a client’s options to modify an object’s state A mutator is often designed so that the values of variables can be set only within particular limits For example, the setFaceValue mutator of the Die class restricts the value to the valid range (1 to numFaces )

38. © 2004 Pearson Addison-Wesley. All rights reserved4-38 Outline Anatomy of a Class Encapsulation Anatomy of a Method Graphical Objects Graphical User Interfaces Buttons and Text Fields

39. © 2004 Pearson Addison-Wesley. All rights reserved4-39 Method Declarations Let’s now examine method declarations in more detail A method declaration specifies the code that will be executed when the method is invoked (called) When a method is invoked, the flow of control jumps to the method and executes its code When complete, the flow returns to the place where the method was called and continues The invocation may or may not return a value, depending on how the method is defined

40. © 2004 Pearson Addison-Wesley. All rights reserved4-40 myMethod(); myMethodcompute Method Control Flow If the called method is in the same class, only the method name is needed

41. © 2004 Pearson Addison-Wesley. All rights reserved4-41 doIt helpMe helpMe(); obj.doIt(); main Method Control Flow The called method is often part of another class or object

42. © 2004 Pearson Addison-Wesley. All rights reserved4-42 Method Header A method declaration begins with a method header char calc (int num1, int num2, String message) method name return type parameter list The parameter list specifies the type and name of each parameter The name of a parameter in the method declaration is called a formal parameter

43. © 2004 Pearson Addison-Wesley. All rights reserved4-43 Method Body The method header is followed by the method body char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } The return expression must be consistent with the return type sum and result are local data They are created each time the method is called, and are destroyed when it finishes executing

44. © 2004 Pearson Addison-Wesley. All rights reserved4-44 The return Statement The return type of a method indicates the type of value that the method sends back to the calling location A method that does not return a value has a void return type A return statement specifies the value that will be returned return expression; Its expression must conform to the return type

45. © 2004 Pearson Addison-Wesley. All rights reserved4-45 Parameters When a method is called, the actual parameters in the invocation are copied into the formal parameters in the method header char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } ch = obj.calc (25, count, "Hello");

46. © 2004 Pearson Addison-Wesley. All rights reserved4-46 Local Data As we’ve seen, local variables can be declared inside a method The formal parameters of a method create automatic local variables when the method is invoked When the method finishes, all local variables are destroyed (including the formal parameters) Keep in mind that instance variables, declared at the class level, exists as long as the object exists

47. © 2004 Pearson Addison-Wesley. All rights reserved4-47 Constructors Revisited Recall that a constructor is a special method that is used to set up a newly created object When writing a constructor, remember that:  it has the same name as the class  it does not return a value  it has no return type, not even void  it often sets the initial values of instance variables The programmer does not have to define a constructor for a class

48. © 2004 Pearson Addison-Wesley. All rights reserved4-48 Bank Account Example Let’s look at another example that demonstrates the implementation details of classes and methods We’ll represent a bank account by a class named Account It’s state can include the account number, the current balance, and the name of the owner An account’s behaviors (or services) include deposits and withdrawals, and adding interest

49. © 2004 Pearson Addison-Wesley. All rights reserved4-49 Using the Account class Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); Account acct3 = new Account ("Edward Demsey", 93757, 759.32); acct1.deposit (25.85); double smithBalance = acct2.deposit (500.00); System.out.println ("Smith balance after deposit: " + smithBalance); System.out.println ("Murphy balance after withdrawal: " + acct2.withdraw (430.75, 1.50)); acct1.addInterest(); acct2.addInterest(); acct3.addInterest(); System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3);

50. © 2004 Pearson Addison-Wesley. All rights reserved4-50 Account class public class Account { private final double RATE = 0.035; // interest rate of 3.5% private long acctNumber; private double balance; private String name; //----------------------------------------------------------------- // Sets up the account by defining its owner, account number, // and initial balance. //----------------------------------------------------------------- public Account (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; }

51. © 2004 Pearson Addison-Wesley. All rights reserved4-51 //----------------------------------------------------------------- // Deposits the specified amount into the account. Returns the // new balance. //----------------------------------------------------------------- public double deposit (double amount) { if (amount > 0) balance = balance + amount; return balance; } //----------------------------------------------------------------- // Withdraws the specified amount from the account and applies // the fee. Returns the new balance. //----------------------------------------------------------------- public double withdraw (double amount) { if (amount <= balance) balance = balance - amount ; return balance; }

52. © 2004 Pearson Addison-Wesley. All rights reserved4-52 //----------------------------------------------------------------- // Returns the current balance of the account. //----------------------------------------------------------------- public double getBalance () { return balance; } //----------------------------------------------------------------- // Returns a one-line description of the account as a string. //----------------------------------------------------------------- public String toString () { NumberFormat fmt = NumberFormat.getCurrencyInstance(); return (acctNumber + "\t" + name + "\t" + fmt.format(balance)); }